1. Field of the Invention
The present invention is directed to the use of a combination of at least two stable nitroxide free radical compounds to inhibit the polymerization of ethylenically unsaturated monomers.
2. Description of Related Art
Many ethylenically unsaturated monomers undesirably polymerize at various stages of their manufacture, processing, handling, storage, and use. A particularly troublesome problem is equipment fouling caused by polymerization in the purification stages of the production processes of such monomers. Polymerization, such as thermal polymerization, during their purification results in the loss of the monomer and a loss in production efficiency owing to the deposition of polymer in or on the equipment being used in the purification, the deposits of which must be removed from time to time. Additionally, the formation of soluble polymer leads to loss of monomer, i.e., a lower yield, and an increase in the viscosity of any tars that may be produced. The processing of the tars then requires higher temperature and work (energy cost) to remove residual monomer.
A wide variety of compounds has been proposed and used for inhibiting uncontrolled and undesired polymerization of ethylenically unsaturated monomers. However, these compounds have not been fully satisfactory. Accordingly, there has been a substantial need in the art for improved compositions for inhibiting the polymerization of such monomers during their production and the distillation process for purifying or separating them from impurities, as well as during transport and storage.
Hindered nitroxyl compounds are known to be very active inhibitors of free radical polymerizations of unsaturated monomers such as styrene, acrylic acid, methacrylic acid, and the like.
U.S. Pat. No. 3,163,677 discloses N,N,O-trisubstituted hydroxylamines and N,N-disubstituted nitroxides of the formulae: 
wherein R1, R2, and R3 are each an alkyl radical having 1 to 15 carbon atoms. (As used herein, the designation Nxe2x80x94O* denotes a stable free radical wherein the asterisk is an unpaired electron.) The N,N,O-trisubstituted hydroxylamines can be used to make the N,N-disubstituted nitroxides, which are stable free radicals and are said to be useful as polymerization inhibitors.
U.S. Pat. No. 3,334,103 discloses that nitroxides can be prepared from the corresponding heterocyclic amine wherein the nitrogen atom of the nitroxide group is attached to other than a tertiary carbon of an aliphatic group (i.e., the nitrogen atom forms a part of a heterocyclic nucleus). These nitroxides are said to have useful properties similar to those described for the N,N-disubstituted nitroxides of U.S. Pat. No. 3,163,677.
U.S. Pat. No. 3,372,182 discloses that a great variety of N,N-disubstituted, stable, free radical nitroxides not otherwise readily available can be prepared by a simple and convenient process that comprises pyrolyzing in an inert reaction medium virtually any hydroxylamine that is susceptible to cleavage of the Oxe2x80x94C bond, e.g., tri-t-butylhydroxylamine.
U.K. Patent Number 1,127,127 discloses that acrylic acid can be stabilized against polymerization by the addition thereto of a nitroxide having the essential skeletal structure: 
wherein R1, R2, R3, and R4 are alkyl groups and no hydrogen is bound to the remaining valencies on the carbon atoms bound to the nitrogen. The two remaining valencies that are not satisfied by R1 to R4 or nitrogen can also form part of a ring (e.g., 2,2,6,6 tetramethyl-4-hydroxy-piperidine-1-oxyl).
U.S. Pat. No. 3,422,144 discloses stable, free radical nitroxides of the formula: 
wherein R is selected from the group consisting of tertiary alkyl, aryl, alkaryl, haloaryl, carboxyaryl, alkoxyaryl, alkylthioaryl, pyridyl, and dialkylaminoaryl, and Rxe2x80x2 is tertiary alkyl. These nitroxides are said to be useful as traps for reactive free radicals both in the counting of free radicals and for inhibiting oxidation and free radical polymerization.
U.S. Pat. No. 3,494,930 discloses free radicals of the nitroxide type for use as initiators of free radical reactions, collectors of free radicals, polymerization inhibitors or antioxidants. They are constituted by nitrogenous bicyclic compounds in which one of the bridges comprises solely the nitroxide radical group and, in particular, by aza-9-bicyclo (3,3,1) nonanone-3-oxyl-9, and by aza-9-bicyclo (3,3,1) nonane oxyl-9.
U.S. Pat. No. 3,873,564 discloses compounds and a method for assaying enzymes by adding to a medium containing an enzyme a stable free radical compound having a stable free radical functionality which, when subjected to an enzyme-catalyzed reaction, changes the environment of the free radical functionality. By following the change in the electron spin resonance spectrum as affected by the change in environment, the type of enzyme and the activity of the enzyme can be determined.
The compounds found useful are normally stable nitroxide radicals with an enzyme labile functionality. Other compounds include two cyclic nitroxide containing rings joined by a chain having an enzyme labile functionality.
U.S. Pat. No. 3,966,711 teaches that 2,2,7,7-tetraalkyl- and 2,7-dispiroalkylene-5-oxo-1,4-diazacycloheptanes substituted in the 4-position by mono- or tetravalent radicals are powerful light-stabilizers for organic polymers They are said to possess higher compatibility than their 4-unsubstituted homologues, from which they can be synthesized by reactions known for N-alkylation. Preferred substituents in the 4-position are alkyl, alkylene, alkenyl, aralkyl, and esteralkyl groups. The 1-nitroxyls derived from the imidazolidines by oxidation with hydrogen peroxide or percarboxylic acids are also said to be good light stabilizers.
U.S. Pat. No. 4,182,658 discloses a method for preventing the polymerization of a readily polymerizable vinyl aromatic compound during distillation at elevated temperatures within a distillation apparatus that is subject to an emergency condition, such as a power outage. This method comprises force-feeding a supplemental polymerization inhibitor having a high solubility in the vinyl aromatic compound and a long duration of efficiency into each of the distillation vessels of a conventional distillation apparatus in an amount sufficient to prevent polymerization therein.
European Patent Application 0 178 168 A2 discloses a method for inhibiting the polymerization of an xcex1,xcex2-ethylenically unsaturated monocarboxylic acid during its recovery by distillation by using a nitroxide free radical.
U.S. Pat. No. 4,665,185 discloses a process for the efficient preparation of nitroxyls of sterically hindered amines by the oxidation of the amine using a hydroperoxide in the presence of a small amount of a metal ion catalyst, at moderate temperature for a short period of time, to give the nitroxyl in high yield and purity.
U.S. Pat. No. 5,254,760 teaches that the polymerization of a vinyl aromatic compound, such as styrene, is very effectively inhibited during distillation or purification by the presence of at least one stable nitroxyl compound together with at least one aromatic nitro compound.
U.S. Pat. Nos. 5,545,782 and 5,545,786 disclose that nitroxyl inhibitors in combination with some oxygen reduce the premature polymerization of vinyl aromatic monomers during the manufacturing processes for such monomers. Even small quantities of air used in combination with the nitroxyl inhibitors are said to result in vastly prolonged inhibition times for the monomers.
U.S. Pat. No. 6,136,951 discloses a method for evaluating the polymer growth inhibition ability of a compound or compounds wherein the method comprises: A) weighing a dry seed of insoluble polymer capable of growing via a living polymerization mechanism; B) subjecting the seed of insoluble polymer to conditions capable of causing a living polymer to grow via a living polymerization mechanism; C) drying the seed; and D) weighing the seed a second time to determine any increase in weight; wherein the polymer growth inhibition ability of the compound or compounds is inversely proportional to any increase found in weight of the seed.
U.S. Pat. No. 6,274,683 discloses an improvement in a method for evaluating the efficiency of polymerization inhibitors, wherein the improvement comprises: A) producing dissolved polymer in a solution comprising monomer and at least one inhibitor by means of any test known in the art to be useful for evaluating polymerization inhibitors, B) collecting the polymer-containing solution, C) measuring the degree of polymerization of the monomer in the collected solution, and D) re-subjecting the polymer-containing solution to the test conditions of A).
U.S. Pat. No. 6,300,533 discloses a method for inhibiting the premature polymerization of ethylenically unsaturated monomers comprising adding to said monomers an effective amount of an inhibitor having the structural formula: 
wherein R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl and the 
portion represents the atoms necessary to form a five-, six-, or seven-membered heterocyclic ring, at least one of said atoms being a carbon atom substituted with a primary, secondary, or tertiary amino group.
U.S. Pat. No. 6,344,560 discloses the use of a nitroaromatic compound to enhance the solubility of a nitroxyl compound in an aromatic hydrocarbon solvent.
U.S. Pat. No. 6,403,850 discloses a method for inhibiting the premature polymerization of ethylenically unsaturated monomers comprising adding to said monomers an effective amount of A) at least one stable hindered nitroxyl compound having the structural formula: 
wherein R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl; and X1 and X2 (1) are independently selected from the group consisting of halogen, cyano, COOR7, xe2x80x94Sxe2x80x94COR7, xe2x80x94OCOR7, (wherein R7 is alkyl of 1 to 15 carbon atoms, or (2) taken together, form a ring structure with the nitrogen; and B) at least one phenylenediamine, and, optionally, C) at least one nitroaromatic compound.
European Patent Application 0 765 856 A1 discloses a stabilized acrylic acid composition in which the polymerization of the acrylic acid is inhibited during the distillation process for purifying or separating the acrylic acid as well as during transport and storage. The compositions comprise three components: (a) acrylic acid, (b) a stable nitroxyl radical, and (c) a dihetero-substituted benzene compound having at least one transferable hydrogen (e.g., a quinone derivative such as the monomethyl ether of hydroquinone (MEHQ)). During the distillation process, transport, and storage, components (b) and (c) are present in a polymerization-inhibiting amount. During the distillation process, oxygen (d) is preferably added with components (b) and (c). According to the specification, examples of suitable nitroxide free radical compounds include di-t-butylnitroxide; di-t-amylnitroxide; 2,2,6,6-tetramethyl-piperidinyloxy; 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy; 4-oxo-2,2,6,6-tetramethyl-piperidinyloxy; 4-dimethylamino-2,2,6,6-tetramethyl-piperidinyloxy; 4-amino-2,2,6,6-tetramethyl-piperidinyloxy; 4-ethanoyloxy-2,2,6,6-tetramethyl-piperidinyloxy; 2,2,5,5-tetramethylpyrrolidinyloxy; 3-amino-2,2,5,5-tetramethylpyrrolidinyloxy; 2,2,5,5-tetramethyl-1-oxa-3-azacyclopentyl-3-oxy; 2,2,5,5-tetramethyl-1-oxa-3-pyrrolinyl-1-oxy-3-carboxylic acid; and 2,2,3,3,5,5,6,6-octamethyl-1,4-diazacyclohexyl-1,4-dioxy.
WO 97/46504 concerns substance mixtures containing: (A) monomers containing vinyl groups; and (B) an active amount of a mixture which inhibits premature polymerization of the monomers containing vinyl groups during their purification or distillation and contains: (i) between 0.05 and 4.5 wt %, relative to the total mixture (B), of at least one N-oxyl compound of a secondary amine which has no hydrogen atom at the xcex1-C atoms; and (ii) between 99.95 and 95.5 wt % relative to the total mixture (B), of at least one nitro compound. The publication also discloses a process for inhibiting the premature polymerization of monomers, and the use of mixture (B) for inhibiting the premature polymerization of monomers.
WO 98/02403 relates to inhibiting the polymerization of vinyl aromatic compounds by using a mixture of a phenol and a hydroxylamine. It is said that the process is useful in ethylbenzene dehydrogenation effluent condenser systems and styrene-water separator vent gas compressor systems and that it effectively inhibits polymerization of monomers, preventing the formation of a polymer coating on condenser and compressor equipment, thus reducing the necessity for cleaning of equipment surfaces.
WO 98/14416 discloses that the polymerization of vinyl aromatic monomers such as styrene is inhibited by the addition of a composition of a stable hindered nitroxyl radical and an oxime compound.
WO 98/25872 concerns substance mixtures containing (A) compounds containing vinyl groups; (B) an active amount of a mixture which inhibits premature polymerization of the compounds containing vinyl groups and contains: (i) at least one N-oxyl compound of a secondary amine which does not carry any hydrogen atoms on the xcex1-carbon atoms; and (ii) at least one iron compound; (C) optionally nitro compounds; and (D) optionally co-stabilizers. The publication also discloses a process for inhibiting the premature polymerization of compounds (A) containing vinyl groups, and the use of (B) optionally mixed with nitro compounds (C) and/or co-stabilizers (D) for inhibiting the premature polymerization of radically polymerizable compounds and stabilizing organic materials against the harmful effect of radicals.
CS-260755 B1 is directed to the preparation of 4-substituted-2,2,6,6-tetramethylpiperidine nitroxyls as olefin stabilizers.
SU-334845 A1 is directed to the inhibition of the radical polymerization of oligoester acrylates using iminoxyl radical inhibitors of a given formula.
SU-478838 is directed to the inhibition of the radical polymerization of oligoester acrylates and the prevention of oligomeric peroxides using a binary polymerization inhibitor comprising quinone.
FR 2,761,060 relates to the prevention of premature polymerization of styrene during its production by dehydrogenation of ethylbenzene by injecting into the process effluent a radical inhibitor based on an oxyl-tetramethylpiperidine derivative.
Wang et al., Lanzhou Daxue Xuebao, Ziran Kexueban 23(3): 138-140 (1987) evaluated the inhibition effect of the stable nitroxide free radical, di-t-butylnitroxide, on the polymerization of acrylic acid and styrene monomers by three methods: separation method, reboiling method, and expanding method. The results showed that di-t-butylnitroxide itself has an inhibiting effect, and when it blends with hydroquinone, t-butyl catechol, benzoquinone, the inhibition effect is better than common inhibitors.
The foregoing are incorporated herein by reference in their entirety.
As used herein, the abbreviation TEMPO stands for 2,2,6,6-tetramethyl-1-piperidinyloxy. Thus, 4-amino-TEMPO is 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxy; 4-hydroxy-TEMPO is 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (also known in the art as HTEMPO); 4-oxo-TEMPO is 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy; and so on. As used herein, unless specified otherwise, the amino group of 4-amino-TEMPO can be primary, secondary, or tertiary, e.g., 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxy, 4-(N-alkylamino)-2,2,6,6-tetramethyl-1-piperidinyloxy, or 4-(N,N-dialkylamino)-2,2,6,6-tetramethyl-1-piperidinyloxy.
As mentioned above, hindered nitroxyl compounds are known to be very active inhibitors of free radical polymerizations of unsaturated monomers. The present invention is directed to the discovery that a mixture of at least two stable hindered nitroxyl compounds (e.g., 4-amino-TEMPO and 4-oxo-TEMPO) provides better performance than an equivalent charge of either component alone, as indicated by the slow rate of polymerization after shutoff of feed in a steady state dynamic testing system.
More particularly, the present invention is directed to a method for inhibiting the premature polymerization of ethylenically unsaturated monomers comprising adding to said monomers an effective amount of a mixture comprising at least two different inhibitors having the following structural formula: 
In this formula, R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl; and X1 and X2 (1) are independently selected from the group consisting of halogen, cyano, COOR9 (wherein R9 is alkyl or aryl), amido, xe2x80x94Sxe2x80x94C6H5, xe2x80x94Sxe2x80x94COCH3, xe2x80x94OCOC2H5, carbonyl, alkenyl, or alkyl of 1 to 15 carbon atoms, or (2) taken together, form a ring structure with the nitrogen, preferably of five, six, or seven members.
The present invention is directed preferably to a method for inhibiting the premature polymerization of ethylenically unsaturated monomers comprising adding to said monomers an effective amount of a mixture comprising at least two different inhibitors having the structural formula: 
wherein R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl, and the 
portion represents the atoms necessary to form a five-, six-, or seven-membered heterocyclic ring. Such ring-completing atoms are preferably carbon atoms, but heteroatoms, such as O, N, P, or S, may also be present. It is preferred that one of the inhibitors be 4-amino-2,2,6,6-tetramethylpiperidinyloxy (i.e., 4-amino-TEMPO). As employed herein, the term xe2x80x9caminoxe2x80x9d, where it is used to refer to an amino group in the 4-position of the TEMPO, is intended to include primary, secondary, and tertiary amino groups.
The effective amount of the combination of nitroxyl compounds is typically about 1 to 2,000 ppm, based on the weight of the ethylenically unsaturated monomer, although amounts outside this range may be appropriate depending upon the conditions of use. The amount of the combination of nitroxyl compounds is preferably about 5 to about 1,000 ppm, based on the weight of the ethylenically unsaturated monomer.
In another embodiment, the present invention is directed to a composition comprising:
(a) an ethylenically unsaturated monomer, and
(b) an effective inhibiting amount, to prevent premature polymerization of the ethylenically unsaturated monomer, of a mixture of at least two different inhibitors having the structural formula: 
In this formula, R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl; and X1 and X2 (1) are independently selected from the group consisting of halogen, cyano, COOR11 (wherein R11 is alkyl or aryl), amido, xe2x80x94Sxe2x80x94C6H5, xe2x80x94Sxe2x80x94COCH3, xe2x80x94OCOC2H5, carbonyl, alkenyl, or alkyl of 1 to 15 carbon atoms, or (2) can be taken together to form a ring structure with the nitrogen, preferably of five, six, or seven members.
In a preferred embodiment, the present invention is directed to a composition comprising:
(a) an ethylenically unsaturated monomer, and
(b) an effective inhibiting amount, to prevent premature polymerization of the ethylenically unsaturated monomer, of a mixture of
(i) 1 to 99 percent by weight, based on the total weight of components (i) and (ii), of 4-amino-2,2,6,6-tetramethylpiperidinyloxy, wherein said amino group is a primary, secondary, or tertiary amino group, and, correspondingly,
(ii) 99 to 1 percent by weight of at least one other inhibitor having the structural formula: 
wherein R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl, and the 
portion represents the atoms necessary to form a five-, six-, or seven-membered heterocyclic ring.
The preferred amounts of components (i) and (ii) are 10 to 90 percent by weight of component (i) and, correspondingly, 90 to 10 percent by weight of component(s) (ii); more preferably, 20 to 80 percent by weight of component (i) and, correspondingly, 80 to 20 percent by weight of component(s) (ii); and most preferably, 20 to 50 percent by weight of component (i) and 50 to 80 percent by weight of component(s) (ii).
In a highly preferred embodiment, the present invention is directed to a method for inhibiting the premature polymerization of ethylenically unsaturated monomers comprising adding to said monomers from about 1 to about 2000 ppm, based on the weight of the ethylenically unsaturated monomer, of a mixture comprising
A) a first inhibitor comprising 4-amino-2,2,6,6-tetramethylpiperidinyloxy, wherein said amino group is a primary, secondary, or tertiary amino group; and
B) at least one other inhibitor having the structural formula: 
wherein:
R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl;
R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl;
X1 and X2 (1) are independently selected from the group consisting of alkyl groups of 1 to 15 carbon atoms or (2) taken together, form a ring structure with the nitrogen.
In another highly preferred embodiment the present invention is directed to a method for inhibiting the premature polymerization of ethylenically unsaturated monomers comprising adding to said monomers from about 1 to about 2000 ppm, based on the weight of the ethylenically unsaturated monomer, of a mixture comprising
A) a first inhibitor comprising 4-amino-2,2,6,6-tetramethylpiperidinyloxy, wherein said amino group is a primary, secondary, or tertiary amino group; and
B) at least one other inhibitor having the structural formula: 
wherein R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl, and the 
portion represents the atoms necessary to form a five-, six-, or seven-membered heterocyclic ring.
In still another highly preferred embodiment the present invention is directed to a composition comprising:
(a) an ethylenically unsaturated monomer, and
(b) an effective inhibiting amount, to prevent premature polymerization of the ethylenically unsaturated monomer, in the range of from about 1 to about 2000 ppm, based on the weight of the ethylenically unsaturated monomer, of a mixture of
(i) a first inhibitor comprising 4-amino-2,2,6,6-tetramethylpiperidinyloxy, wherein said amino group is a primary, secondary, or tertiary amino group; and
(ii) at least one other inhibitor having the structural formula: 
wherein:
R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl;
R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl;
X1 and X2 (1) are independently selected from the group consisting of alkyl of 1 to 15 carbon atoms or (2) taken together, form a ring structure with the nitrogen.